1. Field
Example embodiments relate to a semiconductor memory device, and more particularly to a nonvolatile memory device outputting analog signal and a memory system having the same.
2. Description of Related Art
Semiconductor memory devices are generally the most inevitable micro electronic parts for digital logic devices, such as a computer and an application based on a microprocessor ranging from a satellite to a consumer electronic product. Therefore, improving manufacturing technology of semiconductor memory devices including process and technique development that is acquired through scale-down for high integration and high speed has helped to establish performance criteria of other digital logic devices.
Semiconductor memory devices are largely divided into volatile semiconductor memory devices and nonvolatile semiconductor memory devices. In volatile memory devices, logic information is stored by setting logic state of bi-stable flip-flop in case of SRAM and by charging a capacitor in case of DRAM. Also volatile memory devices can store and read data only while power is supplied but lose data when power is removed. On the other hand, nonvolatile memory devices including MROM, PROM, EPROM, EEPROM, etc can store data even when power is removed. Data state in nonvolatile memory devices can be stored permanently or reprogrammed according to manufacturing process. Reprogrammable nonvolatile semiconductor memory devices have been used to store data and program code in wide range of applications including computer, avionics, telecommunication, and consumer electronic technique industry.
Flash memory is one of example of reprogrammable nonvolatile semiconductor memory devices. As a result of a recent demand for high density of semiconductor memory devices is, multi-bit memory devices that can store multi-bit per memory cell have been developed. In order to store multi-bit in a memory cell of multi-bit flash memory, more threshold voltages distinguishable each other should be included within a limited voltage window. Due to this requirement, an error during read operation caused by variation of threshold voltages can often occur in the multi-bit flash memory. There are various causes of the variation of threshold voltage of multi-bit memory cell. For instance, because of charge leakage of floating gate, program disturbance stress, and read disturbance stress, threshold voltage of the multi-bit memory cell may be increased or decreased by each data state. Accordingly, in multi-bit memory device to store more and more data in per memory cell, a technique for solving the error during read operation has been requested.